The present invention relates to a method of forming an airtight seal structure to be used in an optical fiber guiding portion of an optical device having optical fibers.
In general, an optical device such as semiconductor optical module, LiNbO.sub.3 optical waveguide device and light activated device, which is used in an optical fiber communication device, is encapsulated airtightly by a casing of such as anti-corrosive stainless steel, in order to prevent an optical semiconductor element or optical waveguide element from being influenced by moisture. Further, in order to completely sealing the whole casing externally, it is also necessary to airtightly seal an optical fiber guiding portion of the casing.
In order to completely protect the optical semiconductor element, etc., against external moisture, airtightness in the order of 10.sup.-8 atm.cc/sec is required. When the casing is composed of a metal case body and a metal cover thereof, airtightness in such order can be relatively easily achieved since the seam welding technique can be applied and airtight seal of electrode terminal portion is also easily achieved by fixing a lead portion thereof by glass.
For an optical fiber guiding portion thereof, airtight seal is difficult since the optical fiber is made of silicon oxide which is fragile and easily broken. Since the optical fiber has a protective sheath of resin, it is impossible to apply the seam welding directly. Therefore, it is customary to airtight seal such portion by fixing the sheath of the optical fiber with using adhesive resin (this will be referred as "first" method). With this first method, however, the airtightness obtainable is as low as on the order of 10.sup.-5 atm.cc/sec, which is not enough to obtain an acceptable reliability of the optical device.
As a second method for achieving high airtightness of the optical fiber guiding portion, Japanese Patent Application Laid-open No. Sho 57-68937 discloses a technique in which an optical fiber is covered by a metal material such as gold and the metal cover is soldered to a surrounding metal pipe. This technique makes it possible to obtain an airtight seal of substantially the same degree of the seam weld technique and, therefore, a high reliability of optical device can be obtained. In this case, however, the metal cover or coating of the optical fiber is indispensable and a vapor-deposition of such metal coating on an optical fiber is relatively complicated, leading to a high manufacturing cost.
As a third technique, a direct fixing of an optical fiber to a metal pipe by means of a low melting point glass has been proposed. It is known that the melting point of such low melting point glass is about 500.degree. C. With such high temperature, an optical fiber may be subjected to microbending by which optical characteristics of optical fiber may be degraded.
As a fourth conventional technique, Japanese Patent Application Laid-open No. Hei 5-241028 discloses an optical fiber having a protective coating formed of ultraviolet setting resin. In this technique, when the optical fiber is heated, its optical characteristics is not degraded since there is no thermal stress in the optical fiber although the ultraviolet setting resin is burnt. Therefore, it is possible to airtightly seal the optical fiber without thermal stress. In this technique, however, there may be a case where the optical fiber is partially exposed at a portion on which the resin coating is burnt out.